In history and politics, it's not an Anschluss, it is the "Anschluss".
Literally, the word means connection or the making of a connection. It was used by the Nazis as a euphemism to describe the merger of Austria into Germany (not as a merger of equals, but as a "homecoming" of sorts).
Usage of this word in the context of history and politics is exclusive for this one incident, which is why I put a definite article instead of an indefinite one. It is also advisable to put the term in quotation marks, as I did, to show that you are using Nazi terminology and distance yourself from it.
In all other contexts, the word has no negative connotation and is used often in its literal sense.
Of course, the word can be used to suggest a parallel to the "Anschluss" of Austria in 1938 even in other contexts, but that would be poor taste and the jibe might also go unnoticed. For instance, calling the wedding of two people an "Anschluss" would suggest that one party would give up sovereignty and submit to the whims of the other - not a nice thing to say, and not a nice way to say it.